iCloud & iCloud Photos · Troubleshooting iCloud space
Why is iCloud Drive missing from the Finder sidebar?
Short answer: It's hidden or turned off. Tick iCloud Drive in Finder → Settings → Sidebar, and check iCloud Drive is on in System Settings → your name → iCloud. Go → iCloud Drive in Finder opens it either way.
Either it's hidden or iCloud Drive is off. In Finder, open Finder → Settings → Sidebar and tick iCloud Drive under iCloud. If it isn't offered, check System Settings → your name → iCloud that iCloud Drive is turned on for this Mac.
You can also open it with Go → iCloud Drive in Finder. See turning on iCloud Drive.